Linear motion, such as needed for ground transportation, requires the implementation of (1) propulsion, (2) suspension, and (3) guidance. In conventional wheel-on-rail systems these functions are performed by the wheel through the mediation of friction. If slipping between the driving wheels and the rail is to be prevented, the following inequality must be satisfied between the traction effort or thrust T and the weight W of the locomotive: T.ltoreq..alpha.W, where .alpha., the coefficient of friction decreases with increasing speed. This inequality sets a practical upper limit of about 300 miles/hour for conventional wheel-on-rail roads.
Linear electric motors in which one of the parts in relative motion is carried by the vehicle, while the other lies straight along the track develop three force components: the one in the direction of motion provides the tractive effort needed for propulsion, the other two components perpendicular to the direction of motion, while present, are usually too weak to fulfill the remaining two functions of the wheel, suspension and guidance.
The idea of associating electromechanical power conversion with linear motion dates back to Faraday in the early 1830's, but its recent popularity is largely due to the promotional effort of Prof. E. R. Laithwaite of the Imperial College in London. His motors are of the induction type and utilize iron cores to carry the magnetic flux. Motors of the synchronous types have been adopted for further development in Germany and Japan. The German system utilizes iron cores, while the Japanese one makes use of air cores and superconducting magnets. All three systems are energized by the wayside and the two parts in relative motion are separated by planar air gaps.
The main disadvantage of iron-cored motors is the need to maintain an airgap clearance not exceeding three-eights of an inch. At high speeds this is difficult to achieve and implies high track-maintenance costs. The main disadvantage of the synchronous mode of operation is the need to maintain a perfect match between the vehicle speed and the frequency of the energy supply system and this requires expensive power-conditioning apparatus extending over the whole length of the track.
The air-cored, induction, magnetic levitation system which forms the object of this patent overcomes both these drawbacks, in that being air-cored, it allows an air-gap clearance of a few inches, and being operated in the induction mode allows energization by means of constant, industrial-frequency supplies. Moreover, its special topology with cylindrical air gap develops force components which are strong enough to provide suspension and guidance.
In accordance with the present invention, the energized part of the motor or primary consists of an array of coaxial circular coils or of a helically wound cylindrical solenoid. The primary is divided into sections which are energized sequentially by a plural phase or polyphase system of alternating currents. Thereby producing a traveling wave of magnetic flux density. This flux is coupled to the passive part of the motor or secondary, which ideally consists of a cylindrical sleeve made of conducting material, such as aluminum, located concentric and exterior to the primary.
The relative motion, or slip between the wave traveling along the primary and the secondary induces purely azimuthal currents in the secondary sleeve. The interaction between the primary and secondary currents creates a longitudinal force component used for propulsion and a strong radial centering force component used for levitation and guidance. However, in order to allow for mechanical support of the interior primary, the exterior cylindrical secondary sleeve must be cut along a longitudinal generatrix. In this case, the currents induced in the secondary maintain their azimuthal direction over most of the cylindrical surface and turn longitudinal in the proximity of the cut to close along its two brims through outwards extended ribs.
These longitudinal portions of the secondary currents being in opposite direction in the two brims and having no counterpart in the primary produce no appreciable force and, therefore, degrade only minimally the performance of the ideal motor in its propulsion, levitation, and guidance functions.
The decision on whether to locate the energized primary by the wayside or on board the conveyance depends on economic considerations. Energization from the wayside is advantageous when the density of traffic is high. The primary is then energized in blocks of about 5 miles length for the sake of efficiency and in order to allow for emergency braking by means of phase reversal. In this case, the conveyance carries only the passive secondary and is, therefore, much cheaper and somewhat lighter. When, instead, the energized primary is carried by the conveyance and the passive secondary lies along the road way, the track is cheaper. However, energization on board the conveyance implies the need for either current collection by catenary or third rail, or the need for prime energy storage and conversion apparatus located on board the conveyance.